(i) Field of the Invention
The present invention relates to method and apparatus for heating a strip of metallic material in a continuous annealing furnace.
(ii) Related Art Statement
As shown in FIG. 8, a typical conventional continuous annealing furnace for continuously annealing a strip of metallic material such as a cold rolled steel sheet, tin plated steel sheet or the like is so constructed that the strip 1 is unreeled from a payoff reel and is then introduced into the furnace via a cleaning tank, looper or the like. The furnace is provided with a plurality of rolls (that are called helper rolls) R in both the upper and lower areas thereof and the strip 1 is subjected to heating or cooling at a temperature in the range of 650.degree. C. to 900.degree. C. dependency on the mechanical properties required for the strip product while it moves up and down in the vertical direction in the area as defined between the upper and lower rolls R. After completion of annealing, the strip acquires metallic properties such as high tensile strength, capability of deep drawing or the like at room temperature.
In recent years requirements have been raised from users for improving the method and apparatus for continuously annealing a strip of metallic material having different thicknesses and widths in accordance with different heat cycles in dependence upon the required mechanical properties of the strip product, because there is the tendency for carrying out production in many forms and small quantities. In the conventional furnace the strip 1 is heated up to an elevated temperature in the heating zone by thermal energy radiation in accordance with the radiant tube system. However, it is pointed out that the conventional furnace has the problem that the temperature of the strip to be heated can not be controlled quickly in response to variations of the heat cycle required for the strip, because the temperature of each of the radiant tubes has a large time constant. For instance, when the thickness of the strip 1 increases, that is, a strip having a thickness more than that of the preceding strip is continuously treated and therefore the thick strip having a large heat capacity moves through the heating zone, there is a necessity for raising the temperature of the radiant tubes to a higher level.
However, due to the fact that the radiant tubes themselves have large time constants in the range of 10 to 20 minutes, the strip 1 can not reach a predetermined temperature within a very short period of time after the intensity of combustion of the burners relative to the radiant tubes is changed.
In the meanwhile it is acceptable to change the line speed of the strip 1. When the line speed of the strip 1 is left unchanged until the preceding thin strip 1 moves past the heating zone of the furnace, it results that the front end part of the following thick strip is insufficiently heated. In practice, it was reported that a part of the strip having a very long length of 2000 to 5000 m was insufficiently annealed.
When the line speed of the following thick strip is reduced to a necessary extent in order to insure that it reaches the required temperature, it results that the temperature of the strip is excessively increased and thereby it is annealed excessively. This leads to the production of a strip which has a softer mechanical property than generally required. Alternatively, when the line speed of the strip is changed to an intermediate level, it is found that the preceding strip becomes softened while a part of the following strip is insufficiently annealed.
On the contray, in the case where the thickness of a strip to be annealed decreases in the course of its moving through the heating zone in the furnace, it is obvious that a reverse phenomenon will be recognized to the foregoing case.
In the past users were generally willing to accept a strip product which was softened to a level above the required mechanical properties from the viewpoint of excellent workability. In recent years, however, automation has been increasingly employed for elastic working processes of metallic plates or like material and this leads to the tendency that metallic material softened in the above-described manner is not always willingly received by users. Thus, products which are uniformly treated have become increasingly important for users. However, this causes the joined area where two strips having different thickness are joined to one another to be subjected to irregular treating for a considerably long distance. Therefore, the conventional annealing method can not be employed. To obviate the above-mentioned problem concerning the joined area where the thickness of the strips varies, a proposal was made that a dummy strip should be interposed between two strips to be annealed and the operating conditions of the furnace were changed accordingly during the movement of the dummy strip through the heating zone. As a result, however, it has been found that the furnace has a reduced treating capability. In the meantime, it is necessary that a large quanity of strips having the same size or material must be continuously annealed in order to operate the furnace at high efficiency. This leads to the necessity that a large quantity of strips must be kept in storage as inventory in an area located in close proximity to the continuous annealing furnace in order to facilitate the operation of the furnace as planned. As a result, the inventory cost increases and moreover production can not be carried out in the required acceptable timing relation.
Further, in the case where a thick strip is shifted to a thin strip in the course of the annealing operation or in the case where thin strip is shifted to a thick strip in the reverse manner, there occurs the following problem, particularly where differences in thickness between adjacent strips is remarkably large. For instance, in the case where a thin strip is shifted to a thick strip, gas having a higher temperature is blown toward the moving strip through gas jet nozzles which are exposed to radiant tubes having lower temperatures immediately after the shifting of the thickness is effected in this way. As a result, a high intensity of thermal stress is generated in the gas jet nozzles and this leads to a fear of causing deformation, damage or the like with the gas jet nozzles.
Generally, the conventional continuous annealing furnace employed for continuously annealing a strip of metallic material is so constructed the preheating zone, heating zone, soaking zone and cooling zone (inclusive excessive aging zone in the case where an excessive aging treatment is required for the strip) are arranged one after another as seen from the inlet side of the furnace. Heating in the preheating zone is achieved by direct heating with the use of exhaust gas which is delivered from the heating zone and the soaking zone or by blowing hot air toward the strip to raise it up to an elevated level by heat exchange with the exhaust gas. Further, heating in the heating zone as well as in the soaking zone is achieved by means of a plurality of radiant tubes. On the other hand, cooling in the cooling zone is achieved in accordance with a roll cooling system, a gas jet cooling system or a cooling tube system. In the meanwhile, the temperature of the strip at the outlet of the heating zone is controlled to reach a target temperature by controlling the line speed in such a manner that the value of (thickness of strip).times.(line speed) is kept constant while the temperature of the heating zone is left unchanged, and when the thickness of a strip is changed to another thickness with the same heat cycle being used during the whole operation. In the case where the existing heat cycle is changed to another one the, temperature of the strip at the outlet of the heating zone is controlled by changing the preset temperature in the heating zone.
However, it was found that the conventional continuous annealing furnace has the drawback that the heating zone has slow heat responsibility relative to the temperature thereof and it takes 20 to 30 minutes when the preset temperature of the heating zone is changed to another one and thus there appears to be a difference in temperature, for instance, 100.degree. C. Accordingly, a material rejection, equivalent to the length of about one coil takes place due to insifficient heating, for instance, when the line speed is held at a level of 300 mpm. This means that there is a necessity for preparing a dummy coil having a length as mentioned above. However, the period of time for in which the dummy coil moves past the heating zone in the furnace does not make a contribution to production and moreover using the dummy coil is not preferable from the viewpoint of saving thermal energy. Further, when such a dummy coil is used in the furnace, extra operations such as welding of the dummy coil before it enters the heating zone, cutting the dummy coil after it leaves and handling of the dummy coil in the area extending from the inlet to the outlet of the heating zone is necessary.
Another drawback of the conventional continuous annealing furnace is that when the thickness of the strip is changed to another thickness with the same heat cycle being employed material rejection takes place in the area located in front of and behind the weld point of the strip, because another line speed can not be quickly determined in response to a change in the thickness of the strip. To obviate the above-mentioned drawback, the temperature of the strip at the outlet of the heating zone is kept within the allowable temperature by limiting the amount the thickness of strip is changed, for instance, to within .+-.15% of the thickness of the preceding strip, whereby rejection due to material failure is inhibited. However, such a countermeasure as mentioned above makes it complicated to design an operation schedule relative to a strip to be annealed and to control the number of coils in a coil storage house.